Directionally solidified castings either columnar grained, as in the Ver Snyder U.S. Pat. No. 3,260,505 or single crystal as in the Piearcey U.S. Pat. No. 3,494,709 are cast in molds in which the temperature gradient during solidification is established by selectively controlling the power input to a plurality of heating coils spaced along the vertical axes of the mold. A similar temperature gradient can also be established by other means including the gradual withdrawal of the heated mold from a chamber which is heated above the melting point of the alloy. The mold is heated to a temperature above the melting temperature of the metal to be poured in order to avoid nucleation other than at the bottom of the mold.
Production of such directionally solidified single crystal castings requires a starter zone in the mold and a crystal selector which restricts the growth to a single crystal into the article portion of the mold. The crystal selector is generally a non-linear passageway from the starter zone to the article portion of the mold. This is usually a helically shaped passage terminating at the base of the article portion of the mold and having an open lower end communicating with the starter portion. The crystal selector comprises a small passage and thereby is limited in the cross sectional area. As a result, the crystal selector is incapable, by itself, of sufficient strength to support the pattern around which the mold is formed to provide a cavity to form the cast article.
In the past, use has been made of cocoons to provide a heat transfer path from the solidifying casting to the chill or heat sink beneath the mold. Although the cocoon provides additional mechanical support to the pattern, its use requires a two stage mold construction which complicates and increases the cost of the mold. However, cocoons are not necessary for heat transfer when the mold is withdrawn from the heated chamber. When the cocoon is not used, the benefits of its added mechanical support are lost and the helix must be large enough to support the pattern portion of the mold or pattern support must be provided by additional mold fixtures. Such construction of increased dimension imposes limitations on metal efficiency and equipment utilization.